1. Field of the Invention
The present invention relates to a method of production of a dielectric ceramic composition and a method of production of an electronic device containing dielectric layers such as a multilayer ceramic capacitor.
2. Description of the Related Art
A multilayer ceramic capacitor is broadly used as a compact, large capacity, high reliability electronic device. A large number are used in electrical equipment and electronics. In recent years, along with the reduction in size and improvement in performance of such equipment, increasingly tough demands are being made for further reduction of size, increase of capacity, lowering of price, and improvement of reliability of such multilayer ceramic capacitors.
A multilayer ceramic capacitor is normally produced by stacking and firing a paste of internal electrodes and a slurry of a dielectric (paste) by the sheet method or printing method. In general, Pd or Pd alloy had been used for such internal electrodes, but Pd is high in price, so relatively inexpensive Ni or Ni alloy is now being used. When forming the internal electrodes by Ni or an Ni alloy, however, if firing in the atmosphere, there is the problem that the electrodes end up oxidizing. Therefore, in general, after the binder is removed, firing is performed at an oxygen partial pressure lower than the equilibrium oxygen partial pressure of Ni and NiO, then the dielectric layers are reoxidized by heat treatment (Japanese Unexamined Patent Publication (Kokai) No. 3-113116 and Japanese Patent No. 2787746).
If firing in a reducing atmosphere, however, the dielectric layers are reduced and the specific resistance ends up becoming smaller. Therefore, a reduction resistant dielectric material which is not reduced even if fired in a reducing atmosphere has been proposed (I. Burn et al., xe2x80x9cHigh Resistivity BaTiO3 Ceramics Sintered in COxe2x80x94CO2 Atmospheresxe2x80x9d, J. Mater. Sci., 10, 633 (1975); Y. Sakabe et al., xe2x80x9cHigh-Dielectric Constant Ceramics for Base Metal Monolithic Capacitorsxe2x80x9d, pn J. Appl. Phys., 20 Supple. 20-4, 147 (1981)).
A multilayer ceramic capacitor using such reduction resistant dielectric materials, however, suffers from the problem of a short high temperature accelerated lifetime of the insulation resistance (IR) and a low reliability. Further, it suffers from the problem that the specific dielectric constant of the dielectric falls along with time. This is particularly remarkable under a DC electric field. If the thickness of the dielectric layers is reduced to make the multilayer ceramic capacitor smaller in size and larger in capacity, the strength of the electric field applied to the dielectric layers when applying a DC voltage becomes larger. Therefore, the change in the specific dielectric constant becomes remarkably larger.
In the standard known as the X7R characteristic set in the EIA standard, the rate of change of the capacity is set within xc2x115% between xe2x88x9255xc2x0 C. to 125xc2x0 C. (reference temperature of 25xc2x0 C.). As a dielectric material satisfying the X7R characteristic, for example, the BaTiO3+SrTiO3+MnO-based composition disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-36170 is known. This composition, however, changes a large degree in capacity along with time under a DC electric field. For example, if a DC electric field of 50 V is applied at 40xc2x0 C. for 1000 hours, the rate of change of the capacity ends up becoming about xe2x88x9210 to xe2x88x9230% or so and therefore the X7R characteristic can no longer be satisfied.
Further, in the standard called the xe2x80x9cB characteristicxe2x80x9d, that is, the temperature characteristic of the capacity (EIAJ standard), the rate of change is set to within xc2x110% between xe2x88x9225 to 85xc2x0 C. (reference temperature of 20xc2x0 C.).
Further, as other reduction resistant dielectric ceramic compositions, the BaTiO3+MnO+MgO disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-71866, the (Ba1xe2x88x92xSrxO)aTi1xe2x88x92yZryO2+xcex1((1-z)MnO+zCoO)+xcex2(1-t)A2O5+tL2O3)+wSiO2 (where Axe2x95x90Nb, Ta, V; Lxe2x95x90Y or a rare earth element) disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-250905, the barium titanate adding BaaCa1-aSiO3 disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-83256, etc. may be mentioned.
However, even with these dielectric ceramic compositions, if the thickness of the dielectric layers is a superthin one of for example less than 4 xcexcm, it is extremely difficult to satisfy all of the properties of the temperature characteristic of the capacity, the change in capacity along with time under a DC electric field, the accelerated lifetime of the insulation resistance, and the drop in capacity under a DC bias. For example, in the compositions disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-250905 and Japanese Unexamined Patent Publication (Kokai) No. 2-83256, the problem arises of a short accelerated lifetime of the insulation resistance and a large drop in capacity under a DC bias.
An object of the present invention is to provide a method of production for obtaining a multilayer ceramic capacitor or other electronic device containing dielectric layers able to satisfy all of the temperature characteristics of capacity, that is, the X7R characteristic (EIA standard) and B characteristic (EIAJ standard), even when the dielectric layers are superthin layers and having a small change in capacity along with time under a DC electric field, a long accelerated lifetime of the insulation resistance, and small drop in capacity under a DC bias. Another object of the present invention is to provide a method of production of a dielectric ceramic composition able to be suitably used as a dielectric layer of a multilayer ceramic capacitor or other electronic device containing dielectric layers having such superior properties.
To achieve the first object, according to a first aspect of the present invention, there is provided a method of production of a dielectric ceramic composition having at least
a main component expressed by a formula BamTiO2+n, wherein m is 0.995xe2x89xa6mxe2x89xa61.010, n is 0.995xe2x89xa6nxe2x89xa61.010, and the ratio of Ba and Ti is 0.995xe2x89xa6Ba/Tixe2x89xa61.010,
a first subcomponent containing at least one compound selected from MgO, CaO, BaO, SrO, and Cr2O3,
a second subcomponent containing at least one compound selected from SiO2, MO (where M is at least one element selected from Ba, Ca, Sr, and Mg), Li2O, and B2O3,
a third subcomponent containing at least one compound selected from V2O5, MoO3, and WO3, and
a fourth subcomponent containing an oxide of R (where R is at least one element selected from Y, Dy, Td, Gd, and Ho), wherein
the ratio of the subcomponents with respect to 100 moles of the main component is
first subcomponent: 0.1 to 3 moles,
second subcomponent: 2 to 12 moles,
third subcomponent: 0.01 to 3 moles,
fourth subcomponent: 0.1 to 10.0 moles (where, the number of moles of the fourth subcomponent is a ratio of R alone),
said method of producing the dielectric ceramic composition comprising the step of:
mixing in said main component at least part of other subcomponents except for said second subcomponent to prepare a pre-calcination powder,
calcining the pre-calcination powder to prepare a calcined powder, and
mixing at least said second subcomponent in said calcined powder to obtain the dielectric ceramic composition having molar ratios of the subcomponents to the main component of the above ratios.
In the method of the present invention, preferably a dielectric ceramic composition further containing a fifth subcomponent containing MnO and having a ratio of the fifth subcomponent to 100 moles of the main component of 0.05 to 1.0 mole is obtained.
Preferably, a dielectric ceramic composition having a molar ratio of the third subcomponent to 100 moles of the main component of 0.01 to 0.1 mole, more preferably 0.01 to less than 0.1 mole, is obtained.
In the present invention, more preferably, the second subcomponent is expressed by (Ba,Ca)xSiO2+x (where x=0.7 to 1.2). The second subcomponent is considered to function as a sintering aid.
When the second subcomponent has a composition expressed by (Ba,Ca)xSiO2+x (where x=0.7 to 1.2), the ratio of Ba and Ca in the second subcomponent may be any ratio. Inclusion of only one is also possible.
Note that in the specification, the oxides comprising the main component and the subcomponents are expressed by stoichiochemical compositions, but the states of oxidation of the oxides may also deviate from the stoichemical compositions. The above ratios of the subcomponents are found by conversion from the amounts of metals contained in the oxides comprising the subcomponents to the oxides of the above stoichiochemical compositions. Further, as the powder materials of the dielectric ceramic composition, it is possible to use the above oxides or their mixtures or composite oxides, but it is also possible to suitably select and mix various compounds forming the above oxides or composite oxides upon firing, such as carbonates, oxalates, nitrates, hydroxides, and organic metal compounds.
In the present invention, a mean particle size of the main component is not particularly limited, but preferably is 0.1 to 0.7 xcexcm, more preferably 0.2 to 0.7 xcexcm.
In the present invention, preferably, the pre-calcination powder is prepared so that the molar ratios of components contained in the pre-calcination powder (Ba+metal element of the first subcomponent)/(Ti+metal element of the fourth subcomponent) is less than 1, or (Ba+metal element of the fourth subcomponent)/(Ti+metal element of the first subcomponent) is over 1, and calcination is performed.
In the present invention, preferably, the first subcomponent is always contained in the pre-calcination powder when preparing the pre-calcination powder.
In the present invention, when the pre-calcination powder contains the material of the fourth subcomponent, the calcination temperature is preferably 500xc2x0 C. to less than 1200xc2x0 C., more preferably 600 to 900xc2x0 C. Further, when the pre-calcination powder does not contain the material of the fourth subcomponent, the calcination temperature is preferably 600 to 1300xc2x0 C., more preferably 900 to 1300xc2x0 C., particularly preferably 1000 to 1200xc2x0 C.
Note that the calcination may also be performed for a plurality of times.
The calcined powder should have at least the second subcomponent mixed in it. If necessary, it is possible to further mix in at least one of the main component, first subcomponent, third subcomponent, fourth subcomponent, and fifth subcomponent in accordance with need. It is sufficient that the composition of the dielectric ceramic composition finally obtained become the above range.
To achieve the second object, according to a second aspect of the present invention, there is provided a method of production of a multilayer ceramic capacitor comprised by alternately stacking interal electrodes comprised of Ni or Ni alloy and dielectric layers, where each of dielectric layers contains, in the molar ratios indicated, BaTiO3: 100 moles, at least one of MgO and CaO: 0.1 to 3 moles, MnO: 0.05 to 1.0 mole, Y2O3: 0.1 to 5 moles, V2O5: 0.01 to 3 moles, and BaaCa1xe2x88x92aSiO3 (where the symbol (a) is a number from 0 to 1): 2 to 12 moles,
characterized by using at least 70 wt % of the material, which is premixed in BaTiO3 at least one of MgO, CaO and a compound forming MgO or CaO upon heat treatment, and pre-calcined at a temperature of 900xc2x0 C. to 1300xc2x0 C., with respect to the entire dielectric material.
To achieve the second object, according to a third aspect of the present invention, there is provided a method of production of a multilayer ceramic capacitor comprised by alternately stacking interal electrodes comprised of Ni or Ni alloy and dielectric layers, where each of dielectric layers contains, in the molar ratios indicated, BaTiO3: 100 moles, at least one of MgO and CaO: 0.1 to 3 moles, MnO: 0.05 to 1.0 mole, Y2O3: 0.1 to 5 moles, V2O5: 0.01 to 3 moles, and BaaCa1xe2x88x92aSiO3 (where the symbol (a) is a number from 0 to 1): 2 to 12 moles,
characterized by using at least 70 wt % of the material, which is premixed in BaTiO3 at least one of MgO, CaO and a compound forming MgO or CaO upon heat treatment, MnO or a compound forming MnO upon heat treatment, Y2O3 or a compound forming Y2O3 upon heat treatment, and V2O5 or a compound forming V2O5 upon heat treatment, and pre-calcined at a temperature of 900xc2x0 C. to 1300xc2x0 C., with respect to the entire dielectric material.
In the second and third aspects of the present invention, the molar ratio of the V2O5 to 100 moles of the BaTiO3 is preferably 0.01 to 0.1 mole, more preferably 0.01 to less than 0.1 mole. Further, in the second and third aspects of the present invention, a mean particle size of the BaTiO3 is preferably 0.2 to 0.7 xcexcm. Note that in the second and third aspects of the present invention, the number of moles of Y2O3 is the number of moles of Y2O3 not the number of moles of Y alone.
In the method of production of a conventional dielectric ceramic composition, the BamTiO2+n and the additives are mixed once to prepare the mixed powder or dielectric paste of the dielectric ceramic composition. With the conventional method, however, segregation of the additives (first to fifth subcomponents) etc. occurs in the dielectric ceramic composition after firing and variations end up occurring in the composition between crystals. Due to this segregation, the dielectric constant and the insulation resistance of the dielectric deteriorate.
According to the present invention, by mixing in the main component at least one of the first subcomponent, third subcomponent, fourth subcomponent, and fifth subcomponent, and calcining, leaving aside the second subcomponent, it is possible to suppress variations in the composition between the crystal grains and as a result to suppress the precipitation of the segregation phase and control the size of the segregation phase. Therefore, according to the present invention, it is possible to produce a dielectric ceramic composition suitable for use for a multilayer ceramic capacitor or other electronic device including dielectric layers satisfying both the X7R characteristic and B characteristic, having little change in the capacity under a DC electric field along with time, having a long accelerated lifetime of the insulation resistance, having a small drop in capacity under a DC electric field, and superior in reliability. This was first discovered by the present inventors.
Further, the dielectric ceramic composition obtained by the method of production of the present invention does not contain an element like Pb, Bi, or Zn which evaporates and scatters, so can be fired even in a reducing atmosphere. Therefore, it becomes possible to use a base metal such as Ni or an Ni alloy as the internal electrodes and possible to reduce the cost.
Further, the dielectric ceramic composition obtained by the method of production according to the present invention satisfies the X7R characteristic and the B characteristic, has little deterioration of the aging characteristic of the capacity and insulation resistance due to application of a DC electric field, and is superior in reliability even in firing under a reducing atmosphere. Therefore, the method of the present invention can be expected to be effective as a technique for suppressing deterioration of the rate of change of temperature of the high temperature region accompanying the increased thinness of multilayer capacitors.
Further, the dielectric ceramic composition obtained by the method of production does not contain Pb, Bi, or other substance, so a product with a small detrimental impact on the environment due to dumping, disposal, etc. after use can be provided.
Further, with the method of production according to the present invention, it is possible to realize a dielectric ceramic composition of a uniform composition with little different phases formed by precipitation of the additives and possible to improve the dielectric constant and insulation resistance of the dielectric ceramic composition. Further, with the method of production of the present invention, it is possible to prevent structural defects occurring incidentally, so it is possible to provide a multilayer ceramic capacitor having a high reliability.
Since it is possible to suppress precipitation of different phases without changing the additive composition, it is possible to easily produce a multilayer ceramic capacitor or other electronic device containing dielectric layers having a capacity-temperature characteristic satisfying the X7R characteristic and B characteristic.
In particular, in the present invention, by making the ratio of the third subcomponent to 100 moles of the main component preferably 0.01 to 0.1 mole, more preferably 0.01 to less than 0.1 mole, the insulation resistance (IR), CR product (product of the dielectric constant and the insulation resistance), breakdown voltage (VB) characteristic, and resistance to drop in capacity under a DC bias (DC-Bias characteristic) are improved.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-250156, filed on Aug. 21, 2000, the disclosure of which is expressly incorporated herein by reference in its entirety.